Short Tails from North Ronaldsay
by Dr June Morris
‘Viruses can provide answers to questions we have never even asked’
What has this to do with sheep? Well this was a
quote from a French virologist who has a special interest in
Retroviruses. Retroviruses which infected sheep millions of years
ago, before breeds of sheep developed, can tell us about origins and
movements of sheep over thousands of years.
Retroviruses are passed down
If these retroviruses infected reproductive cells, viral DNA became incorporated into the sheep’s DNA. These endogenous retroviruses, or ERVs, were then passed on to the offspring. Different sheep breeds have different retrotypes, with two in particular being recognised as very ancient retrotypes.
Studies have focussed on the Jaagsiekte retrovirus in sheep, particularly since the endogenous Jaagsiekte retrovirus which has passed down the generations of sheep has a modern day exogenous Jaagsiekte retrovirus counterpart which causes an infection of the lungs and is passes from sheep to sheep. Dolly, the first cloned sheep became infected with this disease.
Only two Jaagsiekte retrotypes found in
North Ronaldsay sheep
Although sheep usually carry
several Jaagsietke retrotypes,
Chessa et al
. in 2009 found that the Orkney or North Ronaldsay sheep
were characterised by having mainly one of these very ancient
Jaagsiekte retrotypes, together with a small amount of a ‘nordic’
Ancient Sheep moved to Orkney by the
By comparison with the retrotype characteristics of other sheep breeds, it would appear that the Orkney sheep were likely to have reached Orkney by the Russian/Scandinavian route from the area of domestication of sheep in the Near East.
Soay Sheep travelled by a
In contrast, the Soay carries another ancient
retrotype characteristic of the Asiatic and Mediterranean Mouflon
which would suggest a Mediterranean route from the area of sheep
domestication. However, unlike the Orkney sheep, the Soay has a
number of other retrotypes found in a wider range of sheep breeds,
indicating perhaps introgression from other breeds. Thus retrovirus
studies are providing an interesting insight into origins, movements
and ‘purity’ of breeds.
Evidence from Scrapie
Another recent study has
provided us with interesting information on the ancient nature of
sheep breeds and possible introgression. Anke
Wietholter (2009) found that the 246 German Mouflon sheep which
he tested for their Scrapie Prion Protein genotype all had the
ARQ/ARQ genotype. This adds weight to the suggestion that this is the ancient, wild type genotype. Some of you will remember that a few years ago we had to argue very strongly that an ancient breed of sheep, like the North Ronaldsay, would all be expected to have the ancient ARQ/ARQ genotype. To follow the National Scrapie Plan guidelines and kill sheep with this genotype, because it increased their susceptibility to Scrapie, would have destroyed the breed.
North Ronaldsay sheep nearly 100%
We did not test the native sheep here on North
Ronaldsay but those tested in mainland UK showed the frequency of
the ARQ allele was 98.7%, higher than all other breeds tested. Only
one DNA sample from one flock had the ARR allelle. The Soay sheep
showed a lower ARQ frequency of 76.4%, indicating more
introgression from other breeds.
North Ronaldsay sheep are our closest
link to Neolithic sheep
Since it appears that the North Ronaldsay may well
be the ‘purest’ of the ancient breeds of sheep, it is important
that this "genetic treasure" is preserved. Fortunately there
are responsible owners who are careful to avoid
any any crosses with other breeds. Many of these concerned
owners are members of the
North Ronaldsay Sheep